OK, let's work through this together. We're starting with 876 and we're going to multiply it by 15, which means we add 876 to itself 15 times.
Step 1: 0 + 876 = 876
Step 2: 876 + 876 = 1752
Step 3: 1752 + 876 = 2628
Step 4: 2628 + 876 = 3504
Step 5: 3504 + 876 = 4380
Step 6: 4380 + 876 = 5256
Step 7: 5256 + 876 = 6132
Step 8: 6132 + 876 = 7008
Step 9: 7008 + 876 = 7884
Step 10: 7884 + 876 = 8760
Step 11: 8760 + 876 = 9636
Step 12: 9636 + 876 = 10512
Step 13: 10512 + 876 = 11388
Step 14: 11388 + 876 = 12264
Step 15: 12264 + 876 = 13140

So, 876*15 = 13140